A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
deu 18:1THE LORD IS THE PRIESTS' AND THE LEVITES' INHERITANCE. (Deu 18:1-8)
The priests the Levites . . . shall eat the offerings--As the tribe of Levi had no inheritance allotted them like the other tribes but were wholly consecrated to the priestly office, their maintenance was to arise from tithes, first-fruits, and certain portions of the oblations presented on the altar, which God having by express appointment reserved to Himself made over, after being offered, to His ministers.
deu 18:3this shall be the priest's due from the people--All who offered sacrifices of thanksgiving or peace offerings (Lev 7:31-33) were ordered to give the breast and shoulder as perquisites to the priests. Here "the two cheeks" or head and "the maw" or stomach, deemed anciently a great dainty, are specified. But whether this is a new injunction, or a repetition of the old with the supplement of more details, it is not easy to determine.
deu 18:6if a Levite . . . come with all the desire of his mind--It appears that the Levites served in rotation from the earliest times; but, from their great numbers, it was only at infrequent intervals they could be called into actual service. Should any Levite, however, under the influence of eminent piety, resolve to devote himself wholly and continually to the sacred duties of the sanctuary, he was allowed to realize his ardent wishes; and as he was admitted to a share of the work, so also to a share of the remuneration. Though he might have private property, that was to form no ground for withholding or even diminishing his claim to maintenance like the other ministering priests. The reason or principle of the enactment is obvious (Co1 9:13). At the same time, while every facility was afforded for the admission of such a zealous and self-denying officer, this admission was to be in an orderly manner: he was to minister "as all his brethren"--that is, a Gershonite with Gershonites; a Merarite with Merarites; so that there might be no derangement of the established courses.
deu 18:9THE ABOMINATIONS OF THE NATIONS ARE TO BE AVOIDED. (Deu 18:9-14)
thou shalt not learn to do after the abominations of those nations--(See on Lev 18:21; Lev 19:26; Lev 19:31; Lev 20:4). In spite of this express command, the people of Canaan, especially the Philistines, were a constant snare and stumbling block to the Israelites, on account of their divinations and superstitious practices.
deu 18:15CHRIST THE PROPHET IS TO BE HEARD. (Deu 18:15-19)
The Lord thy God will raise up unto thee a prophet--The insertion of this promise, in connection with the preceding prohibition, might warrant the application (which some make of it) to that order of true prophets whom God commissioned in unbroken succession to instruct, to direct, and warn His people; and in this view the purport of it is, "There is no need to consult with diviners and soothsayers, as I shall afford you the benefit of divinely appointed prophets, for judging of whose credentials a sure criterion is given" (Deu 18:20-22). But the prophet here promised was pre-eminently the Messiah, for He alone was "like unto Moses" (see on Deu 34:10) "in His mediatorial character; in the peculiar excellence of His ministry; in the number, variety, and magnitude of His miracles; in His close and familiar communion with God; and in His being the author of a new dispensation of religion." This prediction was fulfilled fifteen hundred years afterwards and was expressly applied to Jesus Christ by Peter (Act 3:22-23), and by Stephen (Act 7:37).
deu 18:19whosoever will not hearken unto my words which he shall speak in my name, I will require it of him--The direful consequences of unbelief in Christ, and disregard of His mission, the Jewish people have been experiencing during eighteen hundred years.